The Creep Property Of P92 Heat-resistant Steel At High Temperature And Low Stresses

P92 heat-resistant steel is used for the material of components of thermal power plants,which are exposed to elevated temperatures for extensive periods.The demand for service life of power plant and nuclear power plant etc is longer than 20 years.This requires structural materials can resist creep deformation at high temperature and low stress.The creep strength at low stress obtained by extrapolation method based on the date of creep deformation at high stress is larger than the true creep strength.The creep strength degradation is decided by micro-structure degradation and the change of creep mechanism.Therefore,the aim of this paper is to clarify the creep mechanism,and the effect of micro-structure degradation and change of creep mechanisms on creep behavior in P92 heat-resistant steel at low stress.The main contents and results of this paper are as follows:?1?Traditional uniaxial creep can not meet the needs of low stress creep research because of its low strain resolution.In this paper,a creep test based on a helical spring with high strain resolution is used,which is based on pure torsional deformation.The Abaqus software was used to simulate the mechanical field of the coil spring creep specimen and to obtain the necessary and sufficient conditions for the pure torsional deformation.The results show that with the increase of the helical spring specimen pitch,the shear strain in the XY axis decreases and the Z axis And the positive strain in the direction increases.When the pitch increases from 2mm to 5mm,the decrease of shear stress in XY axis is less than 10%,and the ratio of positive strain in Z axis is less than 17%.The deformation of specimen can be regarded as pure torsional deformation.?2?In order to study the tissue degradation in the long-term creep process,the subject of the sample thermal aging treatment under different conditions,backscatter scanning electron microscopy images,analysis of the thermal aging process precipitates change,found:precipitated phase M₂₃C₆ in the grain and grain Boundary precipitation,the MX phase is mainly precipitated in the grain;M₂₃C₆ in the aging process,the size growth rate is slow,the average cross-sectional diameter of 8000 hours aging only about 140nm;MX precipitation phase aging process to maintain a stable size,the average cut-off Diameter is basically maintained at 50nm.Laves phase precipitated from the grain boundary during thermal aging and gradually became coarse.When the aging time was between 2000 hours and 3000 hours,the precipitated area fraction of Laves phase tended to be stable at 0.95%,and the size tended to be stable after 5000 hours.The average cut-off diameter of 400nm.?3?In order to study the effect of tissue degradation on low-stress creep properties,the specimens were subjected to thermal aging under different conditions and the high-temperature and low-stress"coil spring creep"test was carried out.The results show that due to the small precipitation increase of Laves precipitates The creep resistance of the material itself results in a creep strength increase of the coil spring specimen aged for one month.?4?In order to study the creep mechanism transformation under low stress of P92 heat-resistant steel,high-stress uniaxial tensile creep and low-stress creep test were carried out on the specimens,and the stress dependence of creep rate,Transient creep property and anelastic recovery phenomenon.The results show that creep stress index is 1 in low stress region,creep stress index is 4 in high stress region and different creep mechanism in high stress region and low stress region;The increase of instantaneous strain value is equal to the decrease of instantaneous strain value caused by stress abatement,which indicates that creep may be controlled by the viscous slip of dislocations.There are two different creep strains found after unloading:Strain and plastic strain,showing the characteristics of creep elastic deformation.The present study found that Laves precipitation has a significant effect on the creep of P92 heat-resistant steel under high temperature and low stress,which laid the foundation for predicting the long-term creep life under high temperature and low stress of P92 heat-resistant steel and developing new high-performance heat-resisting steel basis.